xs(7) Crossroads I/O, a lightweight messaging layer


#include <xs/xs.h>

cc [flags] files -lxs [libraries]


Crossroads I/O is a library for building scalable and high performance distributed applications. It fits between classic BSD sockets, JMS/AMQP-style message queues, and enterprise message-oriented middleware.

Crossroads I/O extends the standard socket interfaces with features traditionally provided by specialised messaging middleware products. Crossroads sockets provide an abstraction of asynchronous message queues, multiple messaging patterns, message filtering (subscriptions), seamless access to multiple transport protocols and more.

Crossroads I/O provides a native C API for applications. Support for many more languages is provided by the community through language bindings which can be found at the Crossroads website.

This documentation presents an overview of Crossroads concepts, describes how Crossroads abstract standard sockets and provides a reference manual for the functions provided by the Crossroads library.


Before using any Crossroads library functions the caller must initialise a context using xs_init(). The following functions are provided to handle initialisation and termination of a context:

Initialise Crossroads context


Terminate Crossroads context


Set Crossroads context options


Thread safety

A context is thread safe and may be shared among as many application threads as necessary, without any additional locking required on the part of the caller.

The individual sockets within a context are not thread safe --- applications may not use a single socket concurrently from multiple threads.

A socket may be migrated from one thread to another, by issuing a full memory barrier between individual calls on the socket. For example, this means applications can create a socket in one thread with xs_socket() and then pass it to a newly created thread as part of thread initialization via a structure passed as an argument to pthread_create().

Multiple contexts

Multiple contexts may coexist within a single application. Thus, an application can use Crossroads directly and at the same time make use of any number of additional libraries or components which themselves make use of Crossroads.


A Crossroads message is a discrete unit of data passed between applications or components of the same application. Crossroads messages have no internal structure and from the point of view of Crossroads themselves they are considered to be opaque binary data.

Applications using the Crossroads library send and receive messages directly from/to buffers provided by the application, using the Crossroads functions xs_send() and xs_recv().

Alternatively, applications desiring zero-copy messaging and/or reference counted allocation of messages can use the message handling functions described in this section, and send and receive messages using xs_sendmsg() and xs_recvmsg() respectively. These two approaches are interchangeable.

The following functions are provided to work with messages using zero-copy and/or reference-counted allocation of messages:

Initialise a message

xs_msg_init(3) xs_msg_init_size(3) xs_msg_init_data(3)

Release a message


Access message content

xs_msg_data(3) xs_msg_size(3)

Message manipulation

xs_msg_copy(3) xs_msg_move(3)

Retrieve message option



Crossroads sockets present an abstraction of a asynchronous message queue, with the exact queueing semantics depending on the socket type in use. See xs_socket(3) for the socket types provided.

The following functions are provided to work with sockets:

Creating a socket


Closing a socket


Manipulating socket options

xs_getsockopt(3) xs_setsockopt(3)

Creating and modifiying topologies

xs_bind(3) xs_connect(3) xs_shutdown(3)

Sending and receiving messages

xs_send(3) xs_recv(3)

Sending and receiving messages (zero-copy)

xs_sendmsg(3) xs_recvmsg(3)

Input/output multiplexing. Crossroads provides a mechanism for applications to multiplex input/output events over a set containing both Crossroads sockets and standard sockets. This mechanism mirrors the standard poll() system call, and is described in detail in xs_poll(3).


A Crossroads socket can use multiple different underlying transport mechanisms. Each transport mechanism is suited to a particular purpose and has its own advantages and drawbacks.

The following transport mechanisms are provided:

Unicast transport using TCP


Reliable multicast transport using PGM


Local inter-process communication transport


Local in-process (inter-thread) communication transport



The Crossroads library functions handle errors using the standard conventions found on POSIX systems. Generally, this means that upon failure a Crossroads library function shall return either a NULL value (if returning a pointer) or a negative value (if returning an integer), and the actual error code shall be stored in the errno variable.

On non-POSIX systems some users may experience issues with retrieving the correct value of the errno variable. The xs_errno() function is provided to assist in these cases; for details refer to xs_errno(3).

The xs_strerror() function is provided to translate Crossroads-specific error codes into error message strings; for details refer to xs_strerror(3).


The following miscellaneous functions are provided:

Report Crossroads library version



The Crossroads library provides interfaces suitable for calling from programs in any language; this documentation documents those interfaces as they would be used by C programmers. The intent is that programmers using Crossroads from other languages shall refer to this documentation alongside any documentation provided by the vendor of their language binding.


The Crossroads library provides an optional drop-in libzmq compatibility library for ZeroMQ applications. See xs_zmq(7) for documentation on this option.


The Crossroads documentation was written by Martin Sustrik <m[blue][email protected]m[][1]> and Martin Lucina <m[blue][email protected]m[][2]>.


Free use of the Crossroads library software is granted under the terms of the GNU Lesser General Public License (LGPL). For details see the files COPYING and COPYING.LESSER included with the libxs distribution.

As a special exception, the copyright holders of libxs grant you the right to link the library statically with your software. Refer to the end of the COPYING.LESSER file included with the libxs distribution for details.